Voltage-controlled oscillator and integrated circuit device provided with it

ABSTRACT

A voltage-controlled oscillator has a voltage-controlled oscillation circuit that oscillates at a frequency according to a control voltage and a limiter circuit that limits the output of the voltage-controlled oscillator to a predetermined level. This configuration makes it possible to maintain a constant output level irrespective of the oscillation frequency.

This nonprovisional application claims priority under 35 U.S.C. § 119(a)on patent application Ser. No. 2002-319963 filed in JAPAN on Nov. 1,2002, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage-controlled oscillator thatoscillates at a frequency according to a control voltage, and relatesalso to an integrated circuit device provided with such avoltage-controlled oscillator.

2. Description of the Prior Art

Satellite broadcast tuners, cable television tuners, and the likeincorporate a PLL (phase-locked loop) synthesizer circuit as a means forcontrolling the frequency of the local signal, and its oscillationsource is typically realized with a voltage-controlled oscillator thatoscillates at a frequency according to a control voltage. In a casewhere the frequency of the local signal needs to be varied over a widerange, the oscillation source of a PLL synthesizer circuit is realizedwith a voltage-controlled oscillator having a plurality ofvoltage-controlled oscillation circuits switchably connected in parallelwith one another, each voltage-controlled oscillation circuitoscillating in a different frequency range (see Japanese PatentApplication Laid-Open No. S58-136142).

It is true that, with the voltage-controlled oscillator disclosed in thepatent publication mentioned above, it is possible to vary the frequencyof the local signal over a wide range without unnecessarily widening thevariable oscillation frequency range of each voltage-controlledoscillation circuit. This helps minimize lowering of the Q value of theresonance circuit that is included in each voltage-controlledoscillation circuit, and thus helps achieve satisfactory phase noisecharacteristics.

However, the voltage-controlled oscillator configured as described abovehas the disadvantage that the output levels of the individualvoltage-controlled oscillation circuits differ from one another, causingthe output level of the local signal to vary according to the frequency.This may pose a problem in a case where the succeeding-stage circuitrequires the local signal to be fed thereto at a constant level over theentire frequency range. Even in a voltage-controlled oscillator havingonly one voltage-controlled oscillation circuit, the output level of thelocal signal may vary according to the control voltage fed thereto. Thismay pose a similar problem as described above.

Moreover, in the voltage-controlled oscillator configured as describedabove, the variable oscillation frequency ranges of adjacentvoltage-controlled oscillation circuits are so set as to overlap attheir ends to achieve oscillation over a continuous frequency range as awhole. However, no complete study has ever been made of the fact thatthe variable oscillation frequency ranges of the individualvoltage-controlled oscillation circuits vary owing to various factors(such as variation in the supply voltage, variation in the operatingtemperature, and fabrication variations). As a result, in a case wherethe voltage-controlled oscillator configured as described above isincorporated in an integrated circuit device, it may be impossible touniquely decide which voltage-controlled oscillation circuit to selectfor oscillation at a given frequency.

Now, the problem mentioned above will be explained in more detail withreference to FIG. 7. FIG. 7 is a diagram showing the variableoscillation frequency range of a conventional voltage-controlledoscillator. In the voltage-controlled oscillator shown in this figure,the variable oscillation frequency ranges of its constituentvoltage-controlled oscillation circuits VCO1 and VCO2 are, when mostdeviated on the low side (in a low state), from 90 MHz to 140 MHz andfrom 130 MHz to 180 MHz, respectively, and, when most deviated on thehigh side (in a high state), from 110 MHz to 160 MHz and from 150 MHz to200 MHz, respectively. In this way, in this conventionalvoltage-controlled oscillator, the variable oscillation frequency rangesof the voltage-controlled oscillation circuits VCO1 and VCO2 are so setas to overlap at their ends in each state to achieve oscillation over acontinuous frequency range as a whole.

It is true that, with the voltage-controlled oscillator configured asdescribed above, it is possible to control the oscillation frequency inthe range from 110 MHz to 180 MHz in any state. However, when thevoltage-controlled oscillation circuits VCO1 and VCO2 are consideredindividually, their variable oscillation frequency ranges free from theinfluence of variations are limited to from 110 MHz to 140 MHz and from150 MHz to 180 MHz, respectively. Thus, for oscillation in the frequencyrange from 140 MHz to 150 MHz, which to choose between thevoltage-controlled oscillation circuits VCO1 and VCO2 cannot be uniquelydecided. As a result, the voltage-controlled oscillator configured asdescribed above requires a circuit for checking whether or not each ofthe voltage-controlled oscillation circuits VCO1 and VCO2 can oscillateat a desired frequency and a circuit for choosing, when one of them isfound to be unable to oscillate at that frequency, the other. Thisincreases the circuit scale of and the power consumption by thevoltage-controlled oscillator.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide avoltage-controlled oscillator that can maintain a constant output levelall the time irrespective of the oscillation frequency and to provide anintegrated circuit device provided with such a voltage-controlledoscillator. It is a second object of the present invention to provide avoltage-controlled oscillator that can uniquely decide whichvoltage-controlled oscillation circuit to select for oscillation at agiven frequency and to provide an integrated circuit device providedwith such a voltage-controlled oscillator.

To achieve the first object above, according to one aspect of thepresent invention, a voltage-controlled oscillator is provided with: avoltage-controlled oscillation circuit that oscillates at a frequencyaccording to a control voltage, and a limiter circuit that limits theoutput of the voltage-controlled oscillator to a predetermined level.Alternatively, according to another aspect of the present invention, avoltage-controlled oscillator is provided with: a plurality ofvoltage-controlled oscillation circuits that oscillate at a frequencyaccording to a control voltage; a selector circuit that selects one ofthe voltage-controlled oscillation circuits and makes the selectedvoltage-controlled oscillation circuit operate; and a limiter circuitthat limits the output of the selected voltage-controlled oscillationcircuit to a predetermined level.

To achieve the second object above, according to still another aspect ofthe present invention, a voltage-controlled oscillator is provided with:a plurality of voltage-controlled oscillation circuits that oscillate ata frequency according to a control voltage; and a selector circuit thatselects one of the voltage-controlled oscillation circuits and makes theselected voltage-controlled oscillation circuit operate. Here, thevariable oscillation frequency ranges of adjacent voltage-controlledoscillation circuits are so set as to overlap at their ends, and thevariable oscillation frequency ranges of the individualvoltage-controlled oscillation circuits are so adjusted that the upperend frequency of the nth (where n≧1) voltage-controlled oscillationcircuit as observed when most deviated on the low side is higher thanthe lower end frequency of the mth (where m=n+1) voltage-controlledoscillation circuit as observed when most deviated on the high side.

According to a further aspect of the present invention, an integratedcircuit device is provided with a voltage-controlled oscillatorconfigured as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanying drawings inwhich:

FIGS. 1A and 1B are block diagrams of voltage-controlled oscillatorsembodying the present invention;

FIG. 2 is a circuit diagram of the voltage-controlled oscillator of afirst embodiment of the invention;

FIG. 3 is a circuit diagram of the voltage-controlled oscillator of asecond embodiment of the invention;

FIGS. 4A and 4B are block diagrams showing examples of the layout of thevoltage-controlled oscillation circuits VCO1 and VCO2 and the limitercircuit LMT;

FIGS. 5A and 5B are block diagrams showing examples of the layout of thesupply power lines to the voltage-controlled oscillation circuits andthe limiter circuit;

FIG. 6 is a diagram showing the variable oscillation frequency range ofa voltage-controlled oscillator embodying the invention; and

FIG. 7 is a diagram showing the variable oscillation frequency range ofa conventional voltage-controlled oscillator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B are block diagrams of voltage-controlled oscillatorsembodying the present invention. As shown in FIG. 1A, avoltage-controlled oscillator 1 a embodying the invention is composed ofa voltage-controlled oscillation circuit VCO that oscillates at afrequency according to a control voltage and a limiter circuit LMT thatlimits the output of the voltage-controlled oscillation circuit VCO to apredetermined level. In the voltage-controlled oscillator 1 a configuredin this way, even when the output level of the voltage-controlledoscillation circuit VCO varies as the oscillation frequency thereofvaries according to the control voltage, so long as the output levelremains higher than a predetermined level, the output level of thelimiter circuit LMT remains constant. With this configuration, it ispossible to maintain a constant output level all the time irrespectiveof the oscillation frequency.

In a case where the oscillation frequency needs to be varied over a widerange, as shown in FIG. 1B, a voltage-controlled oscillator 1 bembodying the invention may be composed of a plurality ofvoltage-controlled oscillation circuits VCO1 to VCOn that oscillate at afrequency according to a control voltage, a selector circuit SLT thatselects one of the voltage-controlled oscillation circuits VCO1 to VCOnto make it operate, and a limiter circuit LMT that limits the output ofthe selected voltage-controlled oscillation circuit to a predeterminedlevel. In the voltage-controlled oscillator 1 b configured in this way,even when the output levels of the voltage-controlled oscillationcircuits VCO1 to VCOn differ from one another, so long as all the outputlevels remain higher than a predetermined level, the output level of thelimiter circuit LMT remains constant. With this configuration, it ispossible to maintain a constant output level all the time irrespectiveof the oscillation frequency.

Now, a practical example of the circuit configuration and operation of avoltage-controlled oscillator configured as described above will bedescribed in more detail with reference to FIG. 2. FIG. 2 is a circuitdiagram of the voltage-controlled oscillator of a first embodiment ofthe invention. As shown in this figure, the voltage-controlledoscillator 1 c of this embodiment is composed of two voltage-controlledoscillation circuits VCO1 and VCO2 of which the oscillation frequency isvariable in different ranges, a selector circuit SLT that selects one ofthe voltage-controlled oscillation circuits VCO1 and VCO2 to make itoperate, and a limiter circuit LMT that limits the output of theselected voltage-controlled oscillation circuit to a predeterminedlevel.

The voltage-controlled oscillation circuit VCO1 is composed of PMOStransistors P11 and P12, NMOS transistors N11 to N14, inductors L11 andL12, and, variable-capacitance diodes C11 and C12. Likewise, thevoltage-controlled oscillation circuit VCO2 is composed of PMOStransistors P21 and P22, NMOS transistors N21 to N24, inductors L21 andL22, and variable-capacitance diodes C21 and C22. The limiter circuitLMT is composed of NMOS transistors N31 to N34, load resistors Rout1 andRout2, and a constant current source I1.

The sources of the transistors P11 and P12 are connected together, andthe node at which they are connected together is connected to one outputterminal of the selector circuit SLT. The drain of the transistor P11 isconnected to the gate of the transistor P12, to one end of the inductorL11, to the cathode of the variable-capacitance diode C11, to the drainof the transistor N11, and to the gates of the transistors N12 and N14.The drain of the transistor P12 is connected to the gate of thetransistor P11, to one end of the inductor L12, to the cathode of thevariable-capacitance diode C12, to the drain of the transistor N12, andto the gates of the transistors N11 and N13. The other ends of theinductors L11 and L12 are connected together. The anodes of thevariable-capacitance diodes C11 and C12 are connected together, and thenode at which they are connected together is connected to the line ofthe control voltage. The sources of the transistors N11 to N14 areconnected together, and the node at which they are connected together isgrounded.

The sources of the transistors P21 and P22 are connected together, andthe node at which they are connected together is connected to the otheroutput terminal of the selector circuit SLT. The drain of the transistorP21 is connected to the gate of the transistor P22, to one end of theinductor L21, to the cathode of the variable-capacitance diode C21, tothe drain of the transistor N21, and to the gates of the transistors N22and N24. The drain of the transistor P22 is connected to the gate of thetransistor P21, to one end of the inductor L22, to the cathode of thevariable-capacitance diode C22, to the drain of the transistor N22, andto the gates of the transistors N21 and N23. The other ends of theinductors L21 and L22 are connected together. The anodes of thevariable-capacitance diodes C21 and C22 are connected together, and thenode at which they are connected together is connected to the line ofthe control voltage. The sources of the transistors N21 to N24 areconnected together, and the node at which they are connected together isgrounded.

The drains of the transistors N13 and N23 are connected together, andthe node at which they are connected together is connected through avoltage conversion resistor Rin1 to the supply power line. The drains ofthe transistors N14 and N24 are connected together, and the node atwhich they are connected together is connected through a voltageconversion resistor Rin2 to the supply power line.

One ends of the voltage conversion resistors Rin1 and Rin2 (i.e.,terminals A and B) are respectively connected to the gates of thetransistors N31 and N32, which serve as the differential input terminalsof the limiter circuit LMT. The drains of the transistors N31 and N32,which serve as the differential output terminals of the limiter circuitLMT, are connected together through the load resistors Rout1 and Rout2,and the node at which they are connected together is connected to thesupply power line. The sources of the transistors N31 and N32 areconnected together, and the node at which they are connected together isconnected to the drain of the transistor N34. The gates of thetransistors N33 and N34 are connected together, and the node at whichthey are connected together is connected to the drain of the transistorN33. The drain of the transistor N33 is connected through the constantcurrent source 11 to the supply power line. The sources of thetransistors N33 and N34 are connected together, and the node at whichthey are connected together is grounded. In this embodiment, the currentmirror circuit formed by the transistors N33 and N34 is given a currentmirror ratio of 1:1. Needless to say, the current mirror circuit may begiven any ratio other than 1:1, for example 1: a (where a is a positiveinteger).

In the voltage-controlled oscillator 1 c configured as described above,whichever of the voltage-controlled oscillation circuits VCO1 and VCO2is chosen by the selector circuit SLT is supplied with a supply voltageso as to be enabled to oscillate. Here, the voltage-controlledoscillation circuits VCO1 and VCO2 respectively oscillate at oscillationfrequencies ƒ1 and ƒ2 given by formulae (1) below. In formulae (1), thevariable L1 represents the inductance of the inductors L11 and L12, thevariable L2 represents the inductance of the inductors L21 and L22, thevariable C1 represents the capacitance of the variable-capacitancediodes C11 and C12, and the variable C2 represents the capacitance ofthe variable-capacitance diodes C21 and C22.

$\begin{matrix}{{f_{1} = \frac{1}{2\;\pi\sqrt{{L1} \cdot {C1}}}},{f_{2} = \frac{1}{2\;\pi\sqrt{{L2} \cdot {C2}}}}} & (1)\end{matrix}$

The variables C1 and C2 vary according to the control voltage, and thismakes it possible to vary the oscillation frequencies 1 and 2 by varyingthe control voltage. Moreover, by appropriately adjusting the variablesL1, L2, C1, and C2, it is possible to make the voltage-controlledoscillation circuits VCO1 and VCO2 capable of oscillating in differentfrequency ranges.

The current outputs of the voltage-controlled oscillation circuits VCO1and VCO2 are added together, and their sum is then converted into avoltage by the voltage conversion resistors Rin1 and Rin2. In a casewhere the current outputs of the voltage-controlled oscillation circuitsVCO1 and VCO2 are low, giving the voltage conversion resistors Rin1 andRin2 high resistances makes it possible to obtain a higher voltage. Theoscillation output thus converted into a voltage is then fed to thedifferential amplifier circuit that is included in the limiter circuitLMT. Here, if the input voltage is so high as to be out of the dynamicrange of the differential amplifier circuit, the differential amplifiercircuit operates in a saturated state, and thus yields a constant outputlevel.

Now, the operation of the limiter circuit LMT will be described in moredetail. When the differential amplifier circuit that is included in thelimiter circuit LMT receives a voltage so high as to be out of itsdynamic range, one of the transistors N31 and N32 turns on and the otherturns off. For example, when the voltage at the terminal A is high andthe voltage at the terminal B is low, the transistor N31 turns on andthe transistor N32 turns off On the other hand, when the voltage at theterminal A is low and the voltage at the terminal B is high, thetransistor N31 turns off and the transistor N32 turns on.

Suppose here that the supply voltage is VDD, that the resistance of theload resistors Rout1 and Rout2 of the differential amplifier circuit isRout, and that the tail current that flows through the transistor N34(i.e., the operation current of the differential amplifier circuit) isI. Then, the voltages obtained at output terminals OUT1 and OUT2 whenthe differential amplifier circuit is operating in a saturated state arerespectively either VDD or VDD−(Rout×I). Accordingly, the differentialoutput amplitude (i.e., the output level of the limiter circuit LMT) is2×Rout×I. Thus, by appropriately setting the load resistance Rout andthe tail current I of the differential amplifier circuit, it is possibleto obtain a desired output level.

Next, a second embodiment of the invention will be described in detailwith reference to FIG. 3. FIG. 3 is a circuit diagram of thevoltage-controlled oscillator of a second embodiment of the invention.The voltage-controlled oscillator 1 d of this embodiment is soconfigured that, when formed in an integrated circuit, it suffers fromminimum variation in its output level even in the presence offabrication variations in the characteristics and constants oftransistors, resistors, and other components. As shown in FIG. 3, thevoltage-controlled oscillator 1 d of this embodiment has largely thesame configuration as that of the first embodiment (see FIG. 2).Therefore, in the following description of this embodiment, such circuitelements as are found also in the first embodiment are identified withthe same reference symbols, and their explanations will not be repeated,with emphasis placed on the features unique to this embodiment(specifically, the circuit configuration of the constant current source11 included in the limiter circuit LMT).

As shown in FIG. 3, in the voltage-controlled oscillator 1 d of thisembodiment, the constant current source 11 included in the limitercircuit LMT is composed of PMOS transistors P31 and P32 and a currentproducing resistor Rbias. The sources of the transistors P31 and P32 areconnected together, and the node at which they are connected together isconnected to the supply power line. The gates of the transistors P31 andP32 are connected together, and the node at which they are connectedtogether is connected to the drain of the transistor P31. The drain ofthe transistor P31 is connected to the line of a bias voltage, and isalso grounded through the current producing resistor Rbias. The drain ofthe transistor P32 is connected to the drain of the transistor N33. Inthis embodiment, the current mirror circuit formed by the transistorsP31 and P32 is given a current mirror ratio of 1:1. Needless to say, thecurrent mirror circuit may be given any ratio other than 1:1, forexample 1: b (where b is a positive integer).

When a bias voltage Vbias is applied to the constant current source I1configured as described above, the tail current I of the differentialamplifier circuit is Vbias/Rbias, and thus the differential outputamplitude of the limiter circuit LMT is 2×Rout×I=2×Rout×(Vbias/Rbias).With this configuration, in which the tail current I of the differentialamplifier circuit not completely fixed but varied according to variationin the current producing resistance Rbias, when the voltage-controlledoscillator 1 d is formed in an integrated circuit, even if the loadresistance Rout of the limiter circuit LMT varies, its influence can becanceled with variation in the current producing resistance Rbias. Thishelps reduce variation in the output level.

In particular, by using as the current producing resistor Rbias a deviceof the same type as the load resistors Rout1 and Rout2 and placing theformer near the latter, it is possible to make the tendency of theirfabrication variations similar and thereby minimize variation in theoutput level.

It is advisable to use a band-gap voltage as the bias voltage Vbiasapplied to the constant current source I1 configured as described above.With this configuration, it is possible to maintain a constant outputlevel not only against fabrication variations but also against variationin the supply voltage and variation in the operating temperature.

Next, the layout in which the voltage-controlled oscillation circuitsVCO1 and VCO2 and the limiter circuit LMT are arranged will be describedwith reference to FIGS. 4A and 4B. FIGS. 4A and 4B are block diagramsshowing examples of the layout of the voltage-controlled oscillationcircuits VCO1 and VCO2 and the limiter circuit LMT.

In a voltage-controlled oscillator embodying the invention, it isadvisable to place the limiter circuit LMT in such a way that the wiringconductors thereto from the voltage-controlled oscillation circuits VCO1and VCO2 are equally long. In FIG. 4A, the limiter circuit LMT is placedin a position equally distant from the voltage-controlled oscillationcircuits VCO1 and VCO2. With this configuration, there is no differencein the degree of attenuation caused in the outputs of thevoltage-controlled oscillation circuits VCO1 and VCO2 by the wiringcapacitance. This ensures that the limiter circuit LMT operates inidentical manners for both the voltage-controlled oscillation circuitsVCO1 and VCO2.

In a case where it is impossible to adopt the above layout, or in a casewhere priority is given to securing an ample margin for saturatedoperation of the limiter circuit LMT, it is advisable to place thelimiter circuit LMT in such a way that the wiring conductors theretofrom the individual voltage-controlled oscillation circuits aredecreasingly long in order of decreasing output levels therefrom. InFIG. 4B, the limiter circuit LMT is placed near the voltage-controlledoscillation circuit that yields the lower output (specifically, here,the voltage-controlled oscillation circuit VCO1). With thisconfiguration, it is possible to make the degree of attenuation causedin the outputs of the voltage-controlled oscillation circuits by thewiring capacitance decreasingly high in order of decreasing outputlevels therefrom. This makes it easer to maintain the predeterminedinput level required for the limiter circuit LMT to operate in asaturated state, and makes it possible to secure an ample margin forsaturated operation of the limiter circuit LMT.

Next, the layout of the supply power lines to the voltage-controlledoscillation circuit and the limiter circuit LMT will be described withreference to FIGS. 5A and 5B. FIGS. 5A and 5B are block diagrams showingexamples of the layout of the supply power lines to thevoltage-controlled oscillation circuit and the limiter circuit LMT. FIG.5A shows the circuit configuration of a voltage-controlled oscillatorembodying the invention, where the supply power line to thevoltage-controlled oscillation circuit VCO and the supply power line tothe limiter circuit LMT are separated from each other. By contrast, FIG.5B shows, for comparison, a circuit configuration where the same supplypower lines are shared between the voltage-controlled oscillationcircuit VCO and the limiter circuit LMT. In FIGS. 5A and 5B, the wiringresistances of the supply power lines are represented by Ra to Rd.

As described above, in a voltage-controlled oscillator embodying theinvention, the supply power line to the voltage-controlled oscillationcircuit VCO and the supply power line to the limiter circuit LMT areseparated from each other. By eliminating common loads on the supplypower lines which are shared between the voltage-controlled oscillationcircuit VCO and the limiter circuit LMT in this way, it is possible toprevent one circuit from affecting the other. Specifically, whereas, inthe circuit configuration shown in FIG. 5B, a voltage drop of(Ra+Rb)×(Ia+Ib) occurs in the voltage-controlled oscillation circuitVCO, in the circuit configuration shown in FIG. 5A, the voltage drop is(Ra+Rb)×Ia, achieving a reduction of (Ra+Rb)×Ib in the voltage drop.Thus, with this configuration, it is possible to avoid deterioration ofthe phase noise characteristics of the voltage-controlled oscillationcircuit VCO resulting from the additional provision of the limitercircuit LMT.

Next, the variable oscillation frequency ranges of the individualvoltage-controlled oscillation circuits will be described with referenceto FIG. 6. FIG. 6 is a diagram showing the variable oscillationfrequency range of a voltage-controlled oscillator embodying theinvention. In the voltage-controlled oscillator shown in this figure,the variable oscillation frequency ranges of its constituentvoltage-controlled oscillation circuits VCO1 to VCO3 are, when mostdeviated on the low side (in a low state), from 90 MHz to 140 MHz, from110 MHz to 160 MHz, and from 130 MHz to 180 MHz, respectively, and, whenmost deviated on the high side (in a high state), from 110 MHz to 160MHz, from 130 MHz to 180 MHz, and from 150 MHz to 200 MHz, respectively.In this way, in this voltage-controlled oscillator embodying theinvention, not only are the variable oscillation frequency ranges of theindividual voltage-controlled oscillation circuits VCO1 to VCO3 so setas to overlap at their ends at any state as conventionally practiced,but also those portions of the variable oscillation frequency ranges ofthe individual voltage-controlled oscillation circuits VCO1 to VCO3which are free from variations (specifically, from 110 MHz to 140 MHz,from 130 MHz to 160 MHz, and from 150 MHz to 180 MHz, respectively) areso set as to overlap at their ends. That is, in this voltage-controlledoscillator embodying the invention, the variable oscillation frequencyranges of the individual voltage-controlled oscillation circuits are soadjusted that the upper end frequency of the nth (where n≧1)voltage-controlled oscillation circuit as observed in the low state ishigher than the lower end frequency of the mth (where m=n+1)voltage-controlled oscillation circuit as observed in the high state.

With the voltage-controlled oscillator configured as described above, inany state, not only is it possible to vary the oscillation frequencyover the range from 110 MHz to 180 MHz, but it is also possible touniquely decide which voltage-controlled oscillation circuit to selectfor oscillation at a given frequency. This eliminates the need for acircuit for checking whether or not each of the voltage-controlledoscillation circuits VCO1 to VCO3 can oscillate at a desired frequencyand a circuit for choosing, when any of them is found to be unable tooscillate at that frequency, another. This helps reduce the circuitscale of and the power consumption by the voltage-controlled oscillator.

As described above, with a voltage-controlled oscillator according tothe present invention and with an integrated circuit device providedwith such a voltage-controlled oscillator, it is possible to maintain aconstant output level all the time irrespective of the oscillationfrequency. Moreover, with a voltage-controlled oscillator according tothe present invention and with an integrated circuit device providedwith such a voltage-controlled oscillator, it is possible to uniquelydecide which voltage-controlled oscillation circuit to select foroscillation at a given frequency.

1. A voltage-controlled oscillator comprising: a voltage-controlledoscillation circuit that oscillates at a frequency according to acontrol voltage; and a limiter circuit that, if an output of thevoltage-controlled oscillator is higher than a predetermined level,limits the output of the voltage-controlled oscillator to thepredetermined level, wherein, even when, according to the controlvoltage, the output of the voltage-controlled oscillator varies alongwith the frequency at which the voltage-controlled oscillator isoscillating, if the output of the voltage-controlled oscillator ishigher than the predetermined level, an output of the limiter circuit iskept at a constant level, wherein the limiter circuit includes adifferential amplifier circuit that receives the output of thevoltage-controlled oscillation circuit and operates in a saturatedstate, wherein the differential amplifier circuit operates from anoperation current produced by a constant current source that produces aconstant current by applying a predetermined voltage to a currentproducing resistor.
 2. The voltage-controlled oscillator according toclaim 1, wherein the current producing resistor is a device of a sametype as a load resistor of the differential amplifier, and is placednear the load resistor.
 3. The voltage-controlled oscillator accordingto claim 1, wherein the predetermined voltage is produced by a band-gapcircuit.
 4. The voltage-controlled oscillator according to claim 1,wherein a supply power line to the voltage-controlled oscillationcircuit and a supply power line to the limiter circuit are separate fromeach other.
 5. A voltage-controlled oscillator comprising: a pluralityof voltage-controlled oscillation circuits that oscillate at a frequencyaccording to a control voltage; a selector circuit that selects one ofthe voltage-controlled oscillation circuits and makes the selectedvoltage-controlled oscillation circuit operate; and a limiter circuitthat, if an output of the selected voltage-controlled oscillator ishigher than a predetermined level, limits the output of the selectedvoltage-controlled oscillation circuit to the predetermined level,wherein, even when outputs of the plurality of voltage-controlledoscillation circuits have different levels, if the output of theselected voltage-controlled oscillator is higher than the predeterminedlevel, an output of the limiter circuit is kept at a constant level. 6.The voltage-controlled oscillator according to claim 5, wherein thelimiter circuit includes a differential amplifier circuit that receivesthe output of the voltage-controlled oscillation circuit and operates ina saturated state.
 7. The voltage-controlled oscillator according toclaim 6, wherein the differential amplifier circuit operates from anoperation current produced by a constant current source that produces aconstant current by applying a predetermined voltage to a currentproducing resistor.
 8. The voltage-controlled oscillator according toclaim 7, wherein the current producing resistor is a device of a sametype as a load resistor of the differential amplifier, and is placednear the load resistor.
 9. The voltage-controlled oscillator accordingto claim 7, wherein the predetermined voltage is produced by a band-gapcircuit.
 10. The voltage-controlled oscillator according to claim 5,wherein the limiter circuit is so placed that wiring conductors theretofrom the individual voltage-controlled oscillation circuits are equallylong.
 11. The voltage-controlled oscillator according to claim 5,wherein the limiter circuit is so placed that wiring conductors theretofrom the individual voltage-controlled oscillation circuits aredecreasingly long in order of decreasing output levels from thevoltage-controlled oscillation circuits.
 12. The voltage-controlledoscillator according to claim 5, wherein a supply power line to thevoltage-controlled oscillation circuits and a supply power line to thelimiter circuit are separate from each other.
 13. A voltage-controlledoscillator comprising: a plurality of voltage-controlled oscillationcircuits that oscillate at a frequency according to a control voltage;and a selector circuit that selects one of the voltage-controlledoscillation circuits and makes the selected voltage-controlledoscillation circuit operate, wherein variable oscillation frequencyranges of adjacent voltage-controlled oscillation circuits are so set asto overlap at ends thereof, and variable oscillation frequency ranges ofthe individual voltage-controlled oscillation circuits are so adjustedthat an upper end frequency of an nth (where n≧1) voltage-controlledoscillation circuit as observed when most deviated on a low side ishigher than a lower end frequency of mth (where m=n+1)voltage-controlled oscillation circuit as observed when most deviated ona high side.
 14. An integrated circuit device comprising: avoltage-controlled oscillator including a voltage-controlled oscillationcircuit that oscillates at a frequency according to a control voltageand a limiter circuit that, if an output of the voltage-controlledoscillator is higher than a predetermined level, limits the output ofthe voltage-controlled oscillator to the predetermined level, wherein,even when, according to the control voltage, the output of thevoltage-controlled oscillator varies along with the frequency at whichthe voltage-controlled oscillator is oscillating, if the output of thevoltage-controlled oscillator is higher than the predetermined level, anoutput of the limiter circuit is kept at a constant level, wherein thelimiter circuit includes a differential amplifier circuit that receivesthe output of the voltage-controlled oscillation circuit and operates ina saturated state, wherein the differential amplifier circuit operatesfrom an operation current produced by a constant current source thatproduces a constant current by applying a predetermined voltage to acurrent producing resistor.
 15. An integrated circuit device comprising:a voltage-controlled oscillator including a plurality ofvoltage-controlled oscillation circuits that oscillate at a frequencyaccording to a control voltage, a selector circuit that selects one ofthe voltage-controlled oscillation circuits and makes the selectedvoltage-controlled oscillation circuit operate, and a limiter circuitthat, if an output of the selected voltage-controlled oscillator ishigher than a predetermined level, limits the output of the selectedvoltage-controlled oscillation circuit to the predetermined level,wherein, even when outputs of the plurality of voltage-controlledoscillation circuits have different levels, if the output of theselected voltage-controlled oscillator is higher than the predeterminedlevel, an output of the limiter circuit is kept at a constant level. 16.An integrated circuit device comprising: a voltage-controlled oscillatorcomprising a plurality of voltage-controlled oscillation circuits thatoscillate at a frequency according to a control voltage and a selectorcircuit that selects one of the voltage-controlled oscillation circuitsand makes the selected voltage-controlled oscillation circuit operate,wherein variable oscillation frequency ranges of adjacentvoltage-controlled oscillation circuits are so set as to overlap at endsthereof, and variable oscillation frequency ranges of the individualvoltage-controlled oscillation circuits are so adjusted that an upperend frequency of an nth (where n≧1) voltage-controlled oscillationcircuit as observed when most deviated on a low side is higher than alower end frequency of an mth (where m=n+1) voltage-controlledoscillation circuit as observed when most deviated on a high side.